One type of conventional infrared target acquisition device uses a gimbaled rotating optical system with a reflective telescope whose optical axis is parallel to, but offset from, the axis of rotation of the optical system. The target image is projected onto a single cooled detector through a spoke-like reticle coaxial with the axis of rotation. The reticle is positioned on the gimbal and moves with it, but the detector is off the gimbal and stationary. Because the detector is mounted off the gimbal, it is difficult to effectively shield it from ambient thermal radiation. If the target is on the axis of rotation, the detector sees infrared radiation pulses of a constant frequency. If the target drifts off-axis, a frequency modulation becomes superimposed upon the pulses. This frequency modulation is demodulated and used to move the gimbals so as to realign the axis of rotation with the target. Similar mechanisms also exist that use an amplitude modulation reticle.
Another conventional type of target acquisition device uses a gimbaled but non-rotating optical system and an array of photodetectors (e.g. 128.times.128) in which the location of the target image with respect to the center of the optical axis can be electronically determined.
The rotating type of device is electronically much simpler than the array type, but it is less sensitive since it employs fewer individual detectors to cover the same field of view.
Prior art in this technology includes U.S. Pat. No. 3,353,022 to Schwartz which describes a non-tracking IR search system with a reticle separate from the detector; U.S. Pat. No. 3,872,308 to Hopson et al. which uses a light pipe approach to relay light energy from a reticle to a separate detector; U.S. Pat. No. 3,944,167 to Figler et al. which shows a multidetector reticle arrangement to sort out a target from decoys; U.S. Pat. No. 4,561,611 to Sinclair et al. which concerns a fixed target seeker for a spinning projectile in which the reticle and detector are also separate; U.S. Pat. No. 3,076,095 to Becklund in which the reticle is spaced from the detector-carrying cryogenic finger; and U.S. Pat. No. 4,703,167 to Okumura which shows a reticle placed on a visible light detector for electrical signal spectrum-shifting purposes. In all previous implementations the reticle is not cooled, and the detector views radiation either reflected or emitted from the opagque portion of the reticle.